Composite materials are finding more recognition today with the current increases in demand for strong and lightweight materials. This demand is felt in ship structures, both sea and air, as well as in land vehicle structures.
Since the recent development in composites consisting of densely braided net-shaped fiber skeletons, problems have arisen in effective resin impregnation using conventional resin impregnation techniques.
To date the process for the resin impregnation of net-shaped fibrous skeletal composite preforms and the rigidizing therefore has utilized either a vacuum or pressure impregnation technique. In the past the composite was placed in a closed mold which had impregnation and bleed tubing to the mold. Fluid resin was then pumped into the mold under pressure or drawn into the mold using a vacuum. When evidence of resin was seen from the bleed tubes, the impregnation is stopped and the panel cured using a recommended time/temperature cure cycle. This technique has two major limitations. First, the resin used must have a relatively low viscosity (35 to 100 poise at ambient temperature) and be thermally stable. The resin considered in this case is usually of an epoxy or a polyester type having suboptimal mechanical properties typically only requiring a room temperature cure.
The major drawback these low viscosity resins have is their hydrophilic (high water absorption) nature. Consequently structural properties of composites impregnated with these resins are severely degraded in a water/temperature environment. Second, the fiber packing must be loose enough to allow for the resin to flow through the dry fiber preform. This means that the parts fabricated using this technique, require low fiber volume fractions in order for the resin to flow under a vacuum or pressure, and as a result these composites have unsatisfactory mechanical properties. Furthermore, dry preforms impregnated with these conventional techniques can result in composites having undesirable characteristics, e.g. internal voids, surface porosity, etc.